Order this document by AN93/D Prepared by: Ondrej Pauk Industrial System Application Laboratory Roznov, CZ Figure. Low Cost Current Source for Battery Chargers Demonstration Board This paper describes two designs of low cost current sources for battery charger applications based on the LM27ADJ switching stepdown converter and the MC3334 regulator control circuit. INTRODUCTION This design is a highly cost effective.0 A current source for battery chargers with a rectangular constantcurrent, constantvoltage charging characteristic. This feature assures a basic protection against overcharge whose results can range from minor damage to catastrophic failure of the whole system. This circuit was designed to implement additional charge control based either on the microcontroller or on any other charging control unit in the system that operates from NiCd or NiMH batteries. The MC3334 and this board may be used in a wide variety of applications. All functions needed are performed by just two integrated circuits and a small number of external components. This allows a very compact printed circuit board design and a very cost effective solution. The LM27ADJ Easy Switch step down converter allows the system to operate from 8.0 to 40 Vdc, thus allowing direct operation from both 2 and 24 V board voltages used in the automotive industry. In comparison with linear topologies of battery chargers, this circuit provides much better efficiency, especially over a wide input voltage range. General Description Today s most popular rechargeable battery type is NiCd. When overcharged, this type of battery experiences increasing pressure inside the cell. This can cause opening of the cell s vent and release of oxygen. This has a detrimental affect on the battery, although it may still retain some useful capacity. When NiMH batteries are overcharged, they also increase their internal pressure and release some hydrogen, an extremely explosive gas. The schematic diagram is shown in Figure 2. It is a.0 A (maximum) dumb battery charger that uses the LM27ADJ switching converter to step down the input dc voltage, together with the MC3334, which regulates the charging current flowing into the battery. The switching regulator has high efficiency over a wide input voltage range, which allows this design to be universal. Both 2 and 24 V car batteries as well as cheap, poorly regulated, wall adaptors can be used. The term dumb battery charger means that it offers only some basic protective features and the main protective and control functions are maintained by a µp based main control unit inside the PC or the control function of a cellular phone. This concept allows a very compact and cost effective design. Various charge techniques have to be used to accommodate both NiCd and NiMH type batteries. Both NiCd and NiMH batteries can be charged at a high current rate Motorola, Inc. 997
( c rate) up until the charge limit is reached. After that, the battery has to be charged by a much lower current at the so called trickle charge rate. Trickle charging is a continuous low current charging rate that keeps the battery fully charged. While NiCd batteries have a recommended trickle charge rate of about c/0, for NiMH type it is not recommended to exceed a charging rate of c/40. Some battery manufacturers recommend, for their chemistry, pulse charging instead of continuous current charging. This feature can be accomplished by use of the pin of the LM27. Circuit Operation Circuit operation is as follows. When a discharged battery is connected to the charger, the circuit operates as a constant current source. The LM27ADJ buck regulator is used to step down an unregulated dc input voltage. This regulator is capable of providing up to.0 A of charging current.the amount of charging current flowing into the battery is controlled by the MC3334 regulation control circuit. This IC is used to control the feedback loop in either constantcurrent or constantvoltage mode with automatic crossover. The MC3334 features the unique ability to perform both highside and lowside current sensing, each with either internally fixed or externally adjustable threshold level. This feature makes this circuit very universal and ideally suited for use in connection with a microcontroller based intelligent control systems. In the circuit shown in Figure 2, the MC3334 control circuit is configured for highside current sensing. The voltage drop across the sense resistor provides a voltage that is proportional to the charging current. The current regulation threshold Vsen can be adjusted externally (switch S in position 2 ) in the range of 0 V to 200 mv with respect to Pin 4 of. When the switch S is in position, the current regulation threshold level is set internally to 200 mv. Then the regulated current can be calculated as follows: I reg V sen 0.2 is required in those applications where a high peak level of reverse current is possible, if the source outputs are shorted and the diode is not used. The resistor value should be chosen to limit the input current of the internal VCC clamp diode to less than 20 ma. Excessively large values for will degrade the current sensing accuracy. value can be calculated from the following expression: I pk 0.6 0.02 where Ipk is a peak current flowing through the sense resistor. Once the battery voltage reaches a predetermined level, the MC3334 begins to regulate in the constantvoltage mode and the charger starts to regulate the voltage across the battery. This voltage is monitored by Pin of, the Unregulated DC Input Vin = 0 to 40 V C 00 F/0 V Figure 2. Low Cost Switching Regulator Performs ConstantCurrent/ConstantVoltage 3 Cell Charging Function Vin U LM27ADJ 2 400 H 4 Feedback 3 Gnd D N89.0 k Output L C2 330 F/6 V (from ler) Gnd C3 33 nf 8 7 MC3334 0.22 N400 27 R2 6 39 A VO 3 Battery Cells Under Charge 2 3 4 R 0 k VO 2 S Charge Current (from ler) 2 MOTOROLA ANALOG IC APPLICATIONS INFORMATION
noninverting input of the transconductance amplifier inside the MC3334. This voltage is divided by resistor divider R, R2 to the.2 V internally fixed level Vth. By this arrangement the battery charger output voltage threshold can be set. Moreover, in the lowside current sensing configuration (refer to Figure 3) this threshold level can be externally adjusted over a range of 0 to.2 V with respect to the ground at Pin 4. The maximum battery charger output voltage Vreg (the voltage at the point A with respect to Pin 4 of ) can be calculated as follows: V reg V th. R2 R..2. R2 R. The current control loop is closed by connecting Pin 8 of directly to the feedback input of the LM27 (Pin 4 of U). Under normal working conditions this pin is held at.23 V, resistor is added to convert the MC3334 output current to this voltage. The diode protects the batteries against discharge through when the power source U is switched off. For the N400 diode, used in this design, the typical forward voltage drop is 0.9 V. This value must be added to the voltage of three fully charged battery cells in series when the output voltage threshold level is chosen. Capacitor C3 is used for frequency compensation of an internal transconductance amplifier. The circuit shown in Figure 2 provides high efficiency battery charging with protection against short circuit accomplished by the LM27 internal current limiting. Because it is possible to set the output voltage threshold by a simple resistor divider, various types of battery cells, as well as various number of cells, can be charged. Nonetheless, this circuit has some limitations. In the highside current sensing configuration, shown in Figure 2, the VCC Pin 7 of the MC3334 is connected to the output of the power supply circuit. Such a configuration offers the advantage of a common return path for both ICs, the LM27ADJ and the MC3334, but it has also a drawback. The low limit of the MC3334 supply voltage is.9 V. That implies that charging of a single.2 V cell may not be possible (depending on the voltage drop on, if used). Also the current limit in the case of fully discharged cells or shorted output is given only by the internal current limiting of the LM27, as mentioned above. This drawback can be solved by using the circuit shown in Figure 3. Improved Battery Charger This circuit is very similar to the previous one. It uses the same source IC, the LM27 and also the same charger control IC, the MC3334 but now in the lowside current sensing configuration. The second difference is a different connection of the VCC pin of which is the supply voltage pin of the MC3334. Now this VCC pin is connected directly to the unregulated dc input voltage through the supply current limiting resistor R and resistor R6. The use of the coupling capacitor C4 is essential to assure a stable operation of the whole system. C4 transfers the ac part of the LM27 output voltage (or the LM27 output ripple voltage) through the MC3334 VCC Pin 7 and output Pin 8 into the feedback Pin 4 of the LM27. The way the LM27 operates makes this connection necessary. Since the maximum allowable supply voltage of the MC3334 is 8 V, the Zener diode D3 has to be used to clamp the supply voltage of the MC3334to its operating limit when the input voltage exceeds that value. Use of such an arrangement assures that the charging control circuit will always have a supply voltage high enough, even under short circuit conditions at the output of the battery charger. Switch S can be used the same way as in the previous design. Switch S2 is used to alter the output voltage threshold. When S2 is in position, the voltage threshold on Pin is set internally to.2 V and consequently the output voltage threshold can be set only by the resistor divider R, R2. Switching S2 to position 2 allows an external control of the Pin voltage threshold V th in the range of 0 V to.2 V. This feature contributes to the universality of this battery charger. MOTOROLA ANALOG IC APPLICATIONS INFORMATION 3
Figure 3. Low Cost Switching Regulator Performs ConstantCurrent/ConstantVoltage 3 Cell Charging Function. Version with Full ShortCircuit Regulation Capability. Unregulated DC Input Vin = 0 to 40 V C 00 F/0 V Vin U LM27ADJ 2 400 H 4 Feedback 3 Gnd.0 k Output L D N89 C2 330 F/6 V (from ler) Gnd D3 N474A R 60 R6 00 C4 680 nf A 2 N400 VO Threshold Voltage (from ler) 27 C3 33 nf 8 7 MC3334 S2 6 R2 39 3 Battery Cells Under Charge 2 3 4 R 0 k S 0.22 VO 2 Charge Current (from ler) Figure 4. ConstantVoltage/ConstantCurrent Charging Characteristic of the Improved Current Source for Battery Chargers 6 V O, OUTPUT VOLTAGE (V) 4 3 2 0 0 0.4 0.2 0.6 0.8.0 IO, OUTPUT CURRENT (A) Figure 4 shows the V/I charging characteristic of the improved circuit (see schematic in Figure 3). The small voltage drop in the beginning of the V/I characteristic is caused by dynamic resistance of the diode. This phenomenon can be eliminated either by reconnection of the resistor R2 from the point A to the cathode of the diode, or this diode might be replaced by shorting wire. Both actions would cause also another effect on the V/I characteristic of this circuit. When the output voltage drops below approximately.2 V, the output current will fall down accordingly, thus creating a typical foldback V/I characteristic. 4 MOTOROLA ANALOG IC APPLICATIONS INFORMATION
Table. Parts List (Refer to the Circuit Shown in Figure 2) ÁÁÁÁ Component Quantity Value/Rating Description Á ÁÁ R Á 0 kω, /4 W ÁÁ R2 Á 39 kω, /4 W Á Á 27 Ω, /4 W Á ÁÁÁÁ.0 kω, /4 W ÁÁÁÁ 0.22 Ω, /2 W ÁÁÁÁ C Á 00 µf, 0 V ÁÁ Capacitor Electrolytic C2 Á 330 µf, 6 V Á Capacitor Electrolytic C3 Á 33 nf Á Capacitor Ceramic ÁÁÁÁ D.0 A, 40 V Schottky Diode, N89 ÁÁÁÁ.0 A, 00 V Diode, N400 ÁÁÁÁ S Switch L Á 400 µh,.6 A ÁÁ Inductor, 77 48 BV U Á Á IC, LM27ADJ Á Á IC, MC3334 ÁÁÁÁ NOTES:. Inductor L: Inductance Pins 4, 6. Manufacturer: TECH 39 Power Electronic, Tel. 33/468, Fax 33/44090. 2. All tolerances ±0%, unless otherwise specified. Figure. PCB Layout Component Side Gnd V in C D U C2 R V O V O Figure 6. PCB Layout Copper Side L C3 R2 SW Current Performance of the Sample Design (Refer to the Circuit Shown in Figure 2) Input voltage range........... 8.0 to 40 Vdc Battery charging current....... 0.88 A Open output voltage...........4 V Power converter efficiency..... 77%, Vin = 24 V MOTOROLA ANALOG IC APPLICATIONS INFORMATION
Table 2. Parts List (Refer to the Circuit Shown in Figure 3) Component Quantity Á Value/Rating Á Description R Á 0 kω, /4 W Á ÁÁÁÁ R2 39 kω, /4 W ÁÁÁÁ 27 Ω, /4 W Á.0 kω, /4 W ÁÁ R Á 60 Ω,.0 W Á R6 Á 00 Ω, /4 W Á ÁÁÁÁ 0.22 Ω, /2 W ÁÁÁÁ C 00 µf, 0 V Capacitor Electrolytic Á Á C2 Á 330 µf, 6 V ÁÁ Capacitor Electrolytic C3 Á 33 nf Á Capacitor Ceramic C4 Á 680 nf Á Capacitor Ceramic ÁÁÁÁ D.0 A, 40 V Schottky Diode, N89 ÁÁÁÁ.0 A, 00 V Diode, N400 ÁÁÁÁ D3 Á 6 V,.0 W ÁÁ Zener Diode, N474A S Á Á Switch S2 Á Á Switch L Á 400 µh,.6 A Á Inductor, 77 48 BV ÁÁÁÁ U IC, LM27ADJ ÁÁÁÁ IC, MC3334 NOTES:. Inductor L: Inductance Pins 4, 6. Manufacturer: TECH 39 Power Electronic, Tel. 33/468, Fax 33/44090 Á ÁÁÁÁ 2. All tolerances ±0%, unless otherwise specified. Gnd V in Figure 7. PCB Layout Component Side C R D D3 L U R6 C2 C4 C3 R2 R V O V O Figure 8. PCB Layout Copper Side MOTOROLA ISAL Roznov S S2 Current Voltage Performance of the Sample Design (Refer to the Circuit Shown in Figure 3) Input voltage range........... 8.0 to 40 Vdc Battery charging current....... 0.88 A Open output voltage...........4 V Power converter efficiency..... 77%, V in = 24 V 6 MOTOROLA ANALOG IC APPLICATIONS INFORMATION
PostDesign Recommendation In some cases, there is a need to compensate for a small voltage drop on the constantvoltage part of the V/I characteristic of this current source or to compensate a voltage drop across wires leading from the output of the current source to the charged battery. In that case, it is possible to add a resistor between Pin of the MC3334 and the ground of the circuit shown in Figure 3. Conclusion The LM27ADJ stepdown converter and the MC3334 regulator control circuit used in the battery charger applications offer a solution which is simple, highly efficient and cost effective. The two current sources for battery chargers described in this paper can operate directly from both 2 and 24 V board voltages with no change of the electrical connection. This fact makes these applications especially suitable for the automotive industry. REFERENCES. LM27 Data Sheet and Application Note; order from Motorola by LM27/D. 2. MC3334 Data Sheet and Application Note; order from Motorola by MC3334/D. MOTOROLA ANALOG IC APPLICATIONS INFORMATION 7
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